Geoscience Reference
In-Depth Information
bonds are broken as a result of its mineral surface being disturbed. This tiny magni-
tude of free negative charges allows only a very small number of free cations to be
attracted to its surface from an external solution of simple chemical compounds.
Kaolinites originated as thick layers in the past during more than tens of millions
of years ago due to a strong weathering of minerals in a hot and wet tropical climate.
Nowadays, they are continuing to slowly develop in soils of the tropics.
We can compare the crystal lattice of kaolinite to a cluster of slats. One slat is
white and it represents the tetrahedral sheet. The black slat representing the
octahedral sheet is nailed to the white slat. The nails represent here the oxygens on
the top of the T-sheet and the same oxygen belonging to the O-sheet. The thickness
of this double layer slats is about 0.4 nm (0.4 nanometers = 0.0000004 mm). The
free surface of the black slat is covered by the glue that sticks to another white slack
that is nailed to another black slat. This arrangement of slats is repeating many times
in the same sequence and the glue represents the hydrogen bond. Nothing can pen-
etrate between the mutually glued white-black double layers. Inasmuch as nothing
can extend one white-black combination of slats (double layer) from the next com-
bination of slats (double layer), kaolinites do not swell or shrink because water
molecules cannot enter the space between double layers. Water molecules are big-
ger than the space occupied by hydrogen bonds.
Montmorillonite
, having many characteristics quite opposite to those of kaolin-
ite, belongs to the group
smectites
and was named after the Canton de Montmorillon
in France where it was discovered in 1847. Clays containing predominantly mont-
morillonite - or generally smectites - swell when they are wetted and subsequently
shrink when they are dried. The shrinkage is so intensive that a network of big
cracks appears on the surface of a fi eld with soils formed by those smectite clays.
The cracks are often deep and large enough to stick our entire hand into them. A
detailed description of the mineral montmorillonite isolated from this kind of clay
was performed in the third decade of the last (twentieth) century when adequate
instruments were available to make accurate measurements by X-ray diffraction and
electron microscopy. The fi rst instrument works on the principle that each layer
formation behaves like a mirror. Hence, X-rays refl ected by the layer surfaces mani-
fested in bands of different frequencies show distances that are specifi c for a par-
ticular type of clay mineral. Electron transmission microscopes are built on
principles similar to those of ordinary microscopes but transmit very short waves of
electron radiation rather than the long waves of light visible to the human eye.
Because electrons have wavelengths about 100,000 times shorter than those of vis-
ible light, the resolution of an electron microscope is about one million times greater
than that of an ordinary microscope. From today's recently advanced, more sophis-
ticated instruments, such resolution has been remarkably increased by several orders
of magnitude. Instead of glass lenses to concentrate and focus waves of radiation, a
virtual lens is mathematically manipulated to focus electromagnetic rings of radia-
tion. An object placed against the radiation forms a shadow like we form a shadow
during a sunny day. In an ordinary microscope the shadow is observed down to the
size of about 0.01 mm. In an electron microscope the shadow is observed down to
the size of the object - a clay mineral between roughly 0.005 mm (5
μ
m) and 1 nm
